The present invention relates to a laser transponder for disabling a laser-based speed monitor, which is transmitting a monitor laser beam at a moving vehicle by transmitting a jamming laser beam to the laser-based speed monitor.
Laser transponders for optically receiving and emitting information are today widely used in the fields of communication, surveillance and other areas where exchange of information is of interest.
There is however a growing demand for processing the information with a higher reliability of the communication link.
However, the transmission speed, the amounts of data transmitted and the reliability of the link are all closely connected and an improvement in one parameter will reduce the performance of the two others. Further the growing demand for yet smaller electronic circuitry adds even another factor to take into account, since smaller systems creates larger thermal problems.
U.S. Pat. Nos. 5,767,954 and 5,793,477 both describe a system for disabling a laser-based speed monitor. These systems receive and recognize an optical signal from a speed monitor after which they transmit a fixed high-energy signal to disable the speed monitor. However when triggered by a signal from a speed monitor these systems will respond with a high energy signal, covering a wide band of possible frequencies, which will risk overheating the laser diode if not disabled for a period of time afterward, rendering the systems useless for that period.
Moreover, the laser-based speed monitor can easily recognize this type of laser jamming transponders, and thus alerting the user of the speed monitor that this is being jammed. Thus, improvements in these type devices are needed.
A first aspect of the invention is therefore to provide a new and improved laser transponder and method for responding to a received signal capable of working continuously and reliably without overheating the components.
A second aspect of the invention is to provide a laser transponder and a method, which quickly and reliably will transmit the necessary information.
A third aspect of the invention is to provide a laser transponder and a method, which will make use of laser diodes instead of commonly, used IR-diodes.
A fourth aspect of the invention is to provide a laser transponder and a method, which will not be detected by a laser-based speed monitor as a jamming device.
A novel and peculiar feature of the invention includes that the jamming laser beam is in the shape of a jamming signal consisting of jamming pulse trains emitted with a second frequency, where the monitor laser beam is in the shape of a monitor signal consisting of monitor pulse trains emitted with a first frequency.
In an advantageous embodiment of the invention the laser transponder comprises a microprocessor having stored in a database reference values for the first frequency of at least some of a number of known monitor signals, and for each of those reference values also having stored a corresponding value for the second frequency of the jamming signal; an optical receiver for receiving the monitor signal; a monitor signal converter for converting the monitor signal to an electrical monitor signal and sending the signal as input to the microprocessor; a program of the microprocessor for comparing the received electrical monitor signal with the reference values of the database, selecting the corresponding second frequency, or if no reference values is found selecting the first frequency as the second frequency, and generating an electric jamming signal with the selected frequency; a jamming signal converter for receiving the electrical jamming signal from the microprocessor and converting it to the jamming signal; and an optical transmitter for transmitting the jamming signal to the laser-based speed monitor.
The program can also comprise a method wherein the jamming pulse train is emitted in a specific time frame corresponding to a time frame where the laser-based speed monitor expects to receive the reflection of its monitor pulse train.
Further the program can also comprise a method to emit the jamming pulse train such that it is in phase with the monitor pulse train.
Prior uses of laser diodes were limited to very short operating times since the emitted jamming pulse train by earlier methods emitted information in the entire frequency band of all known laser-based speed monitors, thereby emitting a very high energy signal that would overheat laser diodes that were operated continuously.
Furthermore, the emitted high energy jamming pulse train is easily recognized by the laser-based speed monitor as a jamming signal and would thereby alert the user of the laser-based speed monitor accordingly.
Therefore it is also seen in many other applications that IR-diodes have been used. These have the advantage that they work at very low energies and thereby do not overheat. However, such IR-diodes emit a maximum energy at 850 nm and at the operating wavelength of common laser emitting and laser receiving devices, which is at 905 nm, they only emit 20% of the energy emitted at 850 nm, which creates the need for several IR-diodes where you only need one laser diode.
Yet another advantage of making it possible to have the laser diode to work continuously is that it can work with much higher energies than an IR-diode. Typically, an IR-diode emits light with 100 mW where common laser diodes can emit up to 50W and thereby creating a much higher reliability of the communication link.
Accordingly, the invention provides a laser transponder and method that can recognize a laser-based speed monitor and generate a signal that contains enough information to jam the specific speed monitor, thereby avoiding large energy signals and making it possible to use a laser diode continuously and in a way that prevents the laser transponder to be detected by the laser-based speed monitor and without overheating the laser diode.